The present invention relates to integrated circuits and more particularly, to a regulator having a phase compensation circuit.
Constant voltage circuits, such as series regulators, typically include a phase compensation capacitor to compensate for phase shifts in signals circulating through a feedback loop to stabilize operation. Japanese Laid-Open Patent Publication No. 2004-21577 describes a constant voltage circuit incorporating a phase compensation capacitor in an IC chip. The constant voltage circuit illustrated in FIG. 1 of this publication generates, along with an internal phase compensation capacitor, a phase compensation signal that is in accordance with the output current of a control transistor. The constant voltage circuit uses the phase compensation signal to compensate for phase shifts in a feedback signal. The constant voltage circuit also includes a correction circuit that adjusts the phase compensation amount in accordance with operational conditions, such as the output current. This allows for phase compensations to be performed for a wide range of operational conditions. However, with this prior art structure, it is difficult to physically reduce the size of the constant voltage circuit. Generally, the phase compensation capacitor occupies a large area of the constant voltage circuit (IC chip). For example, the phase compensation capacitor may require a capacitance of over 100 pF depending on conditions, such as the load current, load impedance, and transitional response request. Thus, the incorporation of the phase compensation capacitor in the constant voltage circuit hinders miniaturization of the constant voltage circuit.
Japanese Laid-Open Patent Publication No. 2005-100296 describes a method for separating rotation of a phase with an externally connected output stabilization capacitor. In this case, an external phase compensation capacitor is connected to a constant voltage circuit formed on a chip. In the structure shown in FIG. 1 of this publication, to separate the rotation of a phase with the externally connected output stabilization capacitor, a resistor is connected between the capacitor and an output terminal of the constant voltage circuit. To obtain a large phase margin for a phase shift in a feedback signal, the phase compensation circuit compensation compensates for a phase shift with a circuit that combines the resistor and capacitor. This generates a “zero” in the frequency characteristics (gain curve) of the constant voltage circuit. When the output current of the constant voltage circuit increases, a large drop occurs in the output voltage at the resistor of the phase compensation circuit connected to the output terminal of the constant voltage circuit. To compensate for the large drop, the constant voltage circuit detects the drain current (output current) of an output transistor with a current detection transistor and generates a mirror current that is proportional to the output current. The output voltage of the constant voltage circuit increases in accordance with the generated mirror current. This compensates for phase shifts while compensating for the output voltage drop that occurs at the resistor of the phase compensation circuit. In this case, it is preferable that the constant voltage circuit detects the output current with a mirror current that is as small as possible. Thus, the mirror current is generated with an extremely small value with respect to the drain current of the output transistor. For example, the mirror current is generated with respect to the drain current of the output transistor with a current ratio of 1/1,000 or 1/10,000. Thus, for a current mirror of the output transistor and the current detection transistor, it is difficult to maintain offset errors and gain errors within a tolerable range. As a result, it is practically difficult to maintain the phase compensation accuracy with an output current having a wide range while ensuring accuracy of the output voltage.
It would be advantageous to be able to fabricate a small constant voltage circuit that does not have to need a large phase compensation resistor.